1. Field of the Invention
The present invention relates to detectors of infrared radiation and, more specifically, to thermal detectors of infrared radiation.
2. Description of the Related Art
Infrared (IR) detectors are classified into two categories, photonic and thermal. In a photonic IR detector, infrared photons are absorbed to excite electronic transitions and/or generate photocurrent within an IR absorber, usually a semiconductor material having an appropriate bandgap. The excitation changes material properties of the IR absorber such as, for example, electrical conductivity. This change is measured to quantify the amount of absorbed IR radiation. Photonic IR detectors usually operate at very low temperatures, e.g., about 4 K, to suppress thermally induced electronic transitions and/or thermal “dark” current in the IR absorber. As such, photonic IR detectors utilize cryostats and/or complex cooling systems, which make these detectors heavy, bulky, and relatively expensive.
In a thermal IR detector, the energy of absorbed infrared photons is converted into heat, which causes a temperature increase within the detector. This temperature increase is converted into a mechanical or electrical response, which is measured to quantify the amount of absorbed IR radiation. A sensor employed in a thermal IR detector typically has (i) a resistive bolometer, whose electrical resistance changes with temperature, (ii) a pyroelectric material, which exhibits a spontaneous electric polarization change with temperature, (iii) a thermocouple, whose voltage depends on the thermocouple's temperature differential, and/or (iv) a bi-material cantilever, whose shape is sensitive to temperature changes. Currently, IR detectors employing bi-material cantilevers are among the most sensitive thermal IR detectors.
Unlike photonic IR detectors, thermal IR detectors typically (i) do not use cooling and (ii) operate at temperatures normally present in the environment, e.g., about 300 K. As a result, thermal IR detectors can be light, compact, and relatively inexpensive. However, thermal IR detectors have not yet reached the sensitivity levels accessible to their photonic counterparts.